Koike Taichi, Osawa Raiki, Ishida Shintaro, Iwamoto Takeaki
Department of Chemistry, Graduate School of Science, Tohoku University, Aoba-ku, Sendai, 980-8578, Japan.
Angew Chem Int Ed Engl. 2022 May 9;61(20):e202117584. doi: 10.1002/anie.202117584. Epub 2022 Mar 16.
σ-Type 3-electron-2-center (3e-2c) bonds have been extensively studied as one of the key bonding motifs in radical chemistry and some biological systems. "π-Type 3e-2c-bonded species" that contain a 3e-2c π-bond without an underlying σ-bond framework, however, have been unexplored so far both theoretically and experimentally. Herein, we report the synthesis of the first stable π-type 3e-2c-bonded species, a silicon analogue of a bicyclo[1.1.0]butane radical anion. This compound exhibits an extremely long bridgehead Si-Si bond (3.0638(8) Å) and a strong near-IR absorption at 922 nm in solution which arises from a HOMO→SOMO [π(Si-Si)→π*(Si-Si)] transition. DFT calculations revealed a π-type bonding interaction between the two bridgehead silicon atoms with an unpaired electron mainly delocalized across the corresponding π*-type orbital, which introduces a novel bonding motif for constructing π-electron systems.
σ型3电子-2中心（3e-2c）键作为自由基化学和一些生物体系中的关键成键模式之一，已得到广泛研究。然而，迄今为止，在理论和实验方面都尚未探索过含有3e-2c π键且没有潜在σ键框架的“π型3e-2c键合物种”。在此，我们报告了首个稳定的π型3e-2c键合物种的合成，它是双环[1.1.0]丁烷自由基阴离子的硅类似物。该化合物表现出极长的桥头硅-硅键（3.0638(8) Å），并且在溶液中于922 nm处有强烈的近红外吸收，这源于HOMO→SOMO [π(Si-Si)→π*(Si-Si)]跃迁。密度泛函理论计算揭示了两个桥头硅原子之间的π型键合相互作用，其中未成对电子主要离域于相应的π*型轨道上，这为构建π电子体系引入了一种新颖的成键模式。
